The invention is related to a method for manufacturing a composite of a metal oxide infrastructure and a glass infiltrated in pores of the infrastructure to a method for avoiding or reducing stress corrosion in a glass infiltrated metal oxide infrastructure and a glass infiltrated metal oxide structure. Said composite is for instance useful in the field of dental prosthesis.
U.S. Pat. No. 4,772,436 discloses a method for manufacturing such a composite as a ceramic dental prosthesis. This method comprises infiltration at elevated temperatures of a glass in an infrastructure which has been obtained by binding metal oxide particles in a predetermined shape, often the shape of a tooth or a part thereof. This shape is obtained from a model produced in a moulding mass, such as plaster, which exhibits slight linear expansion during solidification. A method of producing an infrastructure in its predetermined shape involves, as described in more detail, preparation of a slip comprising a suspension in water of metal oxide particles such as aluminium oxide and/or zirconia, to which is added a suspension stabilization agent and optionally a pH control agent. The slip is then poured into the molding mass so that the metal oxide particles aggregate. The model with the slip is then baked in order to effect initial dehydration of the model of the tooth which causes its withdrawal. The solid phase of the metal oxide particles is then slightly fritted providing the infrastructure. Usually the metal oxide infrastructure comprises alumina, but may also include magnesium oxide, lanthanum oxide or a rare earth metal oxide. Optionally zirconium oxide particles are used in admixtures in amounts that can be significant. Zirconium oxide may also, in pure form or stabilized with yttrium oxide, be employed to construct the infrastructure.
The infiltration is carried out by placing a glass and the infrastructure in contact with each other and raising the temperature to the infiltration temperature which usually lies within a range from 1000 to 1400° C. The infiltration characteristics are further depending on, amongst others, the heating rate and the time during which the elevated temperature is maintained. The glass preferably exhibits, at the infiltration temperature, characteristics which enable easy wetting of the metal oxide infrastructure, meaning that surface energy of the glass, at the infiltration temperature, must preferably be lower than the surface energy of the infrastructure. The wetting characteristics can be increased by introducing, for instance, lead oxide, boron oxide or vanadium oxide. The reactivity of the glass with regard to the metal oxide must be neither too strong, nor too weak. This is obtained by using a glass which contains initially metal oxides such as alumina and/or zirconia in an amount that is slightly lower than, but close to, the saturation of the glass vis-à-vis those metal oxides at the infiltration temperature. The infiltration temperature can be raised if small amounts of magnesium oxide powder and/or zirconium oxide powders are added to the glass. Also lanthanum oxide or powders of yttrium oxide or rare earth oxides allow for raising the infiltration temperature.
Dental prostheses made of glass infiltrated metal oxide infrastructures are, in use, as a result of for instance chewing, subjected to (cyclic) loading, leading to a phenomenon usually referred to as fatigue. Due to exposure to these conditions, a dental prosthesis, or a part thereof, will ultimately break. The fracture is understood to be a result of slow crack growth of presumably small defects present or flaws in the composite. This slow crack growth may also be referred to as Subcritical Crack Growth (SCG). It is believed that crack growth of these defects or flaws occurs on a microscopic scale by a process of slow diffusion which is enhanced by repeatedly applying stress to the prosthesis and this particular fatigue related wear phenomenon is therefor also referred to as stress corrosion. Due to a process of slow crack growth which results in fracture, the dental prosthesis has a limited lifetime after placement within the set of teeth of a patient.
It is an object of the invention to provide a method for manufacturing a composite of a metal oxide infrastructure and a glass infiltrated in pores of the infrastructure and, to obtain a composite with a higher resistance against slow crack growth compared to composites obtained by methods of manufacturing from the prior art.
The present inventors have found that this aim is reached by applying an extra processing step before infiltration of the glass in the infrastructure.